As a cassette library apparatus of this kind, as shown in FIG. 11, the one in which a plurality of garages 4 are disposed in an annular form on a turn table 2 and a tape cassette 1 is housed in each of the garages 4 is known. In this apparatus, the turn table 2 is rotated by a drive mechanism 3, and thereby, the garage 4 housing the targeted cassette 1 is opposed to an elevator mechanism 5. This elevator mechanism 5 takes out the targeted tape cassette 1 out of the garage 4 and loads and unloads the targeted tape cassette 1 into and from the tape drive 6, whereby readout and write of information are carried out (“New Technology of Matsushita Electric 2002” 2003/05, pages 155 to 159 “DDS loaded backup library” issued by R&D Planning Office, Matsushita Electric Industrial Co., Ltd.).
In this construction, loading and unloading the tape cassettes into and out of the tape drive 6 can be automated within the range of the data library housed in the turn table 2. However, when a number of tape cassettes 1 which are housed in the limited space as the tape library are needed, a magazine type cassette library apparatus shown in FIG. 12 is said to be more suitable than a turn table type cassette library apparatus like this.
In this magazine type cassette library apparatus, the tape drive 6 and a set of magazine type garages 8a and 8b are disposed inside a casing 7 as shown in FIG. 12. A plurality of tape cassettes 1 are housed in each of the garages 8a and 8b in the state in which they are aligned in a row. A picker 9 disposed between the set of garages 8a and 8b transfers the tape cassette 1 from and to these garages 8a and 8b, and the tape drive 6.
FIG. 13 shows a construction of one example of the tape cassette 1. This tape cassette 1 is formed into a thin rectangular parallelepiped shape as shown in the drawing, and recessed parts 10a and 10b are respectively formed at both side portions of its bottom side. Reference numeral 11 denotes a bottom surface of the tape cassette 1, and reference numerals 12a and 12b denote its side surfaces. As shown in FIGS. 12 to 14, the tape cassette 1 is housed in the garages 8a and 8b so that its recessed parts 10a and 10b are located at open sides of the garages 8a and 8b, namely, the sides near to the picker 9. Each of the recessed parts 10a and 10b has a side wall surface 13 which is formed at the open sides of the garages 8a and 8b, namely, the positions near to the picker 9, and a bottom surface 14.
As shown in FIGS. 12 and 15, the picker 9 includes a movable body 15 movable in the horizontal direction. The movable body 15 has a set of swing arms 16a and 16b as shown in FIGS. 12 to 15, and the swing arms 16a and 16b are swingable in an up-and-down direction. Each of the swing arms 16a and 16b has a main body 17 in the horizontal direction and a raised part 18 raised upward from a tip end of the main body 17.
The swing arms 16a and 16b swing upward in the state in which the movable body 15 of the picker 9 approaches the tape cassette 1 housed in the garages 8a and 8b as shown in FIG. 15, and thereby, the main bodies 17 contact the bottom surface 11 of the tape cassette 1, and the raised parts 18 get into the recessed parts 10a and 10b as shown in the drawing. At this time, the raised parts 18 of a set of the swing arms 16a and 16b restrict the positions of the side wall surfaces 13 of the recessed parts 10a and 10b. From the above, the tape cassette 1 is chucked by the swing arms 16a and 16b. In this state, the movable body 15 moves in the direction to be away from the garages 8a and 8b, whereby the raised parts 18 of the swing arms 16a and 16b abut on the side wall surfaces 13 of the recessed parts 10a and 10b, and the tape cassette 1 can be drawn out of the garages 8a and 8b. 
As shown in FIGS. 12 and 15, the picker 9 has a guide rail 19, and the tape cassette 1 which is drawn out is placed on this guide rail 19. Thereby, transfer of the tape cassette 1 from the garages 8a and 8b to the picker 9 is completed.
In order to accurately perform chucking and unchucking of the tape cassette 1 by swing of the swing arms 16a and 16b, the postures of the swing arms 16a and 16b need to be detected in real time.
Therefore, in the prior art, the postures of the swing arms 16a and 16b are detected by sensing several important points in the swing mechanism of the swing arms 16a and 16b. Alternatively, the postures are detected by sensing torque of a motor for driving the swing mechanism of the swing arms 16a and 16b. 